1. Field of the Invention
The invention relates to an image forming apparatus such as a multi-function peripheral, a copier, a printer or a fax machine provided with a fixing part which is provided with a heater heated through power distribution (energization) and which heats a toner image to fix it.
2. Description of Related Art
Typically, a xerographic image forming apparatus (printer, copier, facsimile, or the like) is provided with a fixing part that heats and pressurizes a toner image formed on a sheet to fix it. Moreover, the fixing part is provided with a heater heated through power distribution for the purpose of heating the toner image. A control part or a control unit in the image forming apparatus turns ON/OFF power distribution to the heater to keep the fixing part at an appropriate temperature. Here, for example, a circuit is provided which generates a periodical interrupt signal such as a zero cross signal of an AC voltage applied to the heater. Then for precise control of the power distribution to the heater, based on the interrupt signal, the control part or the like may perform ON/OFF control of the power distribution to the heater. For example, the control part or the like turns ON/OFF the heater with a rising edge or a trailing edge of the interrupt signal as a trigger.
Disclosed is an image forming apparatus which generates a zero cross signal as such an interrupt signal. Specifically, disclosed is an image forming apparatus including: a power source unit outputting a DC voltage based on AC input; an electrical load driven by the AC input; an error detector driven by the DC voltage; an AC input interruption detector detecting AC input interruption; and an error processor that, when an error has been detected, examines with the AC input interruption detector whether or not the AC input is in an interrupted state and ignores this error if the AC input is in the interrupted state, wherein the AC input interruption detector includes: a zero cross signal generator generating a zero cross signal of the AC input; a means adapted to periodically count the zero cross signal; a means adapted to measure time that has passed since a count value of the zero cross signal was cleared to zero; and a means adapted to determine that the measured time has become larger than a predetermined value before the count value of the zero cross signal reaches a predefined value. With this configuration, an attempt is made to prevent occurrence of erroneous error detection following unexpected AC input interruption.
For example, in performing the ON/OFF control of the heater in the fixing part by using the interrupt signal such as the zero cross signal, the interrupt signal may stop (disappear) due to, for example, failure or reckless driving of the circuit generating the interrupt signal. Here, the control part or the like switches ON/OFF of the heater with the rising edge or the trailing edge of the interrupt signal as the trigger. Therefore, the stop of the interrupt signal in a state in which the power distribution to the heater is ON maintains the state that the power distribution to the heater is ON. In that case, the heater continuously heats up and turns into a temperature-excessively-raised state, causing, for example, melting of a member in the fixing part. Then, upon passage of a certain period of time since the disappearance of the interrupt signal, the control part or the like may judge that an error has occurred, thus turning OFF the power distribution to the heater.
On the other hand, in the image forming apparatus, a power source part may be provided. The power source part receives power supply from a commercial power source or the like, performs rectification, stepping down, etc. to generate a plurality of kinds of voltages (for example, DC voltages for driving the control part and for supply to a motor). This power source part typically includes an element, such as a smoothing capacitor or the like, that stores energy; therefore, even when a main power source of the image forming apparatus has been turned OFF, due to discharge from the capacitor or the like, the control part may continue to drive for a certain period of time (for example, several seconds). Moreover, as a result of turning OFF the main power source, the interrupt signal such as the zero cross signal is no longer generated. However, if the control part or the like is still driving at a time point at which the control part or the like has judged the error after the disappearance of the interrupt signal, the control part erroneously detects that an error of interrupt signal stop has occurred. This raises a problem that unnecessary operations, for example, error display on a display part, storage of a history of the error detection into a storage part, etc. are performed.
Since the conventional image forming apparatus described above has the error processor that ignores the error when the AC input interruption has been detected, it seems that the aforementioned problem does not occur. However, the image forming apparatus described above counts the zero cross signal and the time, and when the measured time has become larger than the predetermined value before the count value of the zero cross signal reaches the predefined value, determines the AC input interruption. Therefore, even when the AC input is continuously inputted due to, for example, failure or disconnection in the zero cross signal generator, it is judged as the AC input interruption. In other words, even when the zero cross signal stops due to failure occurring in the zero cross signal generation circuit in a state in which the main power source is ON, the error is ignored. In that case, the power distribution to the heater may be continuously ON, thus causing excessive temperature rise at the fixing part. Therefore, in the conventional image forming apparatus, in terms of safety, a problem worse than the performance of the unnecessary operations arises.